Various studies and proposals have been made relating to a nitrogen oxides (NOx, hereinafter) catalyst for reducing and removing NOx in an exhaust gas of an internal combustion engine such as a diesel engine and certain gasoline engines or various combustion apparatus.
Among these NOx catalysts, a NOx occlusion reduction type catalyst containing NOx occluding substance is used or under consideration to be used for purifying an exhaust gas from a lean-burn gasoline engine or a diesel engine.
Unlike a three-way-catalyst, this NOx occlusion reduction type catalyst can purify NOx even if oxygen (O2, hereinafter) exists in an exhaust gas. FIG. 9 shows the structure of the NOx occlusion reduction type catalyst 70, while FIG. 10 and FIG. 11 show the disposition of active metal on the support layer surface and the mechanism of occlusion, release and removal of NOx.
The NOx occlusion reduction type catalyst 70 shown in FIG. 9 is formed as a monolithic honeycomb 70M provided with a number of polygonal (quadrangular in FIG. 9) cells 70S on a catalyst support 71 of structural material made of codierite, stainless steel and the like.
And, as shown in FIG. 9(b) and FIG. 9(c), the inner wall of the cell 70S presenting a large surface area as a whole is coated with a catalyst coat layer (support layer) 74 formed with a porous coating material such as zeolite or alumina (Al2O3) and silica in order to increase the surface area. And, as shown in FIG. 10 and FIG. 11, a catalyst metal 72 and a NOx occluding substance (R) 73 are supported by the catalyst coat layer 74.
This catalyst metal 72 is formed with a precious metal such as platinum (Pt) having an oxidation function, while the NOx occluding substance (R) 33 is formed with one or several of alkali metals such as potassium (K), alkaline-earth metals such as barium (Ba, hereinafter) or rare-earth such as lanthanum (La) having NOx occlusion and release function.
FIG. 10 shows the mechanism of purification by NOx occlusion with the NOx occlusion reduction type catalyst 70, under an exhaust gas condition where O2 is contained in the exhaust gas such as the exhaust condition in normal operation of a diesel engine, a lean-burn gasoline engine or the likes.
Under this exhaust gas condition, a nitrogen monoxide (NO, hereinafter) in the exhaust gas reacts with O2 in the exhaust gas to be oxidized and turns into nitrogen dioxide (NO2, hereinafter) by the catalysis of the catalyst metal 72. Then, the NOx occluding substance (R) 73 such as Ba occludes the NO2 in a form of nitrate (for instance, Ba(NO3)2) and consequently the NOx in the exhaust gas is purified.
However, if the condition continues, as the NOx occluding substance (R) 73 having a NOx occlusion function is totally transformed into nitrate losing the NOx occlusion function, the engine operation condition is changed to generate a rich state exhaust gas (exhaust gas of a rich air/fuel ratio) without O2 and with a high carbon monoxide (CO, hereinafter) concentration and a high exhaust temperature, then the exhaust gas is sent to the NOx occlusion reduction type catalyst.
Then, as shown in FIG. 11, when O2 becomes absent in the exhaust gas and the CO concentration and the exhaust temperature rise, the NOx occluding substance (R) 73 that has occluded NOx releases NO2, as the nitrite returns into the original form such as Ba. And, since O2 is absent in the exhaust gas, the released NO2 is reduced to water, (H2O, hereinafter), carbon dioxide (CO2, hereinafter) and nitrogen (N2, hereinafter) and purified, reacting with a CO, a hydrocarbon (HC, hereinafter), a hydrogen (H2, herein after) and so on in the exhaust gas as the reducer by the catalysis of the catalyst metal 72.
In a part of these exhaust gas purifying systems of the conventional art provided with the NOx occlusion reduction type catalyst 70, it has been proposed to reduce O2 concentration and increase CO concentration in the exhaust gas with a small quantity of fuel injection, by decreasing the intake quantity through a large quantity of EGR (exhaust gas recirculation), as the exhaust gas purifying system described in the Laid-Open Japanese Patent Publication 2000-356127, in order to avoid the deterioration in fuel efficiency caused by the generation of rich state exhaust gas. In this exhaust gas purifying system, the rich burn is realized with a small quantity of fuel injection, and the deterioration in fuel efficiency is avoided, by performing EGR.
However, in the exhaust gas purifying system of the Laid-Open Japanese Patent Publication 2000-356127, if O2 concentration in the exhaust gas is decreased by a large quantity of EGR, such problems occur that neither the combustion temperature in the engine cylinder nor the temperature of the exhaust gas rises and that inert gases such as CO2 increase on the catalyst surface.
Therefore, another problem of deterioration in the NOx purifying performance appears, that is because the catalyst surface temperature does not rise during the rich-burn accompanied by the EGR leaving the catalyst unactivated and the generated NO2 is released without being removed, although it is necessary to heat the catalyst surface to a high temperature since a large amount of energy is required to reduce NO2 released from the NOx occluding substance to N2.
FIG. 12 shows the NOx purifying performance of a NOx occlusion reduction type catalyst in an exhaust gas purifying system generating an exhaust gas which is in a rich-burn state by using a conventional EGR. According to the FIG. 12, it can be understood that a peak of high NOx concentration appears at the beginning of the regeneration where a rich-burn operation for regenerating a NOx occlusion reduction type catalyst starts by the rich-burn (rich state) accompanied by EGR and that the NOx removal efficiency significantly lowers.
The present invention is devised to solve the aforementioned problems and the object is to provide an exhaust gas purifying system and a method of an exhaust gas purification capable of removing sufficiently NOx released from a NOx occlusion reduction type catalyst and achieving a high NOx removal efficiency, even in a rich-burn accompanied by EGR by executing a catalyst activation control operation to heat up the catalyst surface immediately before the rich-burn accompanied by EGR starts when regenerating a NOx occlusion reduction type catalyst.